Dimmers often are used in homes, theaters and studios, as well as other locations. For example, a lighting fixture containing tungsten lamps can be connected to a dimmer switch on the wall which changes the fixture light output depending on the position of a knob or slider in the dimmer.
In general, the dimmer is connected to an alternating current (AC) line, which provides a voltage that varies with time, typically in the shape of a sine wave. The dimmer modifies the shape of the sine wave to reduce the power delivered to the lamp. Triac, silicon controlled rectifier (SCR) and Insulated Gate Bipolar Transistor (IGBT)-based dimmers accomplish this result by cutting off a portion of the sine wave. Sine wave dimmers achieve this result by reducing the amplitude of the sine wave.
Because of the greater efficiency of light emitting diodes (LEDs), there is movement toward having LED-based light sources replace tungsten lamps. For many applications, this involves using an array of LEDs to obtain the equivalent light output of a tungsten lamp. LEDs are current driven devices and require a minimum voltage for current to flow. Their light output can be changed by changing the current through the device or by rapidly turning the current on and off. The greater the percentage of time the current is on, the greater the amount of light that is produced.
LEDs, however, cannot easily be driven directly by a conventional dimmer (i.e., those designed to be used with a tungsten lamp). For example, LEDs typically require a low DC voltage drive (e.g., 1-5 volts), whereas a conventional dimmer output is a higher AC voltage (e.g., 100-250 volts). If an LED were driven by a conventional dimmer in conjunction with a voltage rectification and reduction circuit, the light output of the combination would not respond to the dimmer changes in the same way as a tungsten lamp.